Among the different possible ways to convert polymers into films, the blown film process with air-cooling is probably the most economical and also the most widely used. This is because films obtained by blowing have a tubular shape which makes them particularly advantageous in the production of bags for a wide variety of uses (e.g. bags for urban refuse, bags used in the storage of industrial materials, for frozen foods, carrier bags, etc.) as the tubular structure enables the number of welding joints required for formation of the bag to be reduced when compared with the use of flat films, with consequent simplification of the process. Moreover, the versatility of the blown-film technique makes it possible, simply by varying the air-insufflation parameters, to obtain tubular films of various sizes, therefore avoiding having to trim the films down to the appropriate size as is necessary in the technique of extrusion through a flat head.
To date the application of polypropylene (PP) for blown film technology has been restricted to niche applications or technologies, such as PP blown film process with water contact cooling ring for highly transparent packaging film and PP used as sealing or temperature resistance layer in multilayer structures. Recently, blown film producers are showing more interest developing new structures with polypropylene. Polypropylene is expected to offer some advantages (e.g. heat resistance, puncture resistance, downgauge) compared to polyethylene. It has been seen that impact copolymers (or heterophasic copolymers) with low melt flow rate, such as ATOFINA PP 4180 polypropylene, have high melt strength and good mechanical properties that enable blown extrusion in monolayer structures with good bubble stability.
Some resin suppliers have patents relating to monolayer and multilayer structures made using impact copolymers. Several applications are mentioned including industrial bags, bags for frozen foods, carrier bags, heavy-duty shipping sacks, among others. There is a constant need for materials having improved properties for particular applications.
It would be desirable if methods could be devised or discovered to provide polypropylene film or sheet materials having improved properties.